With recent progress in the development of solid-state image sensors such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal-Oxide Semiconductor) having a high pixel, digital still cameras and digital video cameras (simply referred to as “digital cameras”, hereinafter) are rapidly spreading that employ an imaging device including an imaging optical system of high optical performance corresponding to the above-mentioned solid-state image sensors of a high pixel. Among these digital cameras having a high optical performance, the demand for the compact digital camera has increased especially.
The compact digital cameras are requested to reduce the further thickness in terms of ease of portability and storability. As means for achieving such thin and compact digital cameras, in conventional art, various kinds of zoom lens systems having a three-unit construction of negative lead type and a short optical overall length (overall length of lens system: the distance from the tip of the most object side lens surface to an image surface in the entire lens system) have been proposed that, in order from an object side to an image side, comprise: a first lens unit having negative optical power; a second lens unit having positive optical power; and a third lens unit having positive optical power.
For example, Japanese Laid-Open Patent Publication No. 2008-241794 discloses a zoom lens, in order from the object side to the image side, having three lens units of a negative lens unit, a positive lens unit, and a positive lens unit. In the zoom lens, intervals between the individual lens units vary at the time of magnification change. Also in the zoom lens, the relationship among focal lengths of the entire system at a wide-angle limit and a telephoto limit, imaging magnification of a second lens unit at a telephoto limit, and a focal length, an Abbe number, a partial dispersion ratio, a refractive index and a radius of curvature of a lens constituting the second lens unit is specified. The zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2008-241794 has a wide view angle at a wide-angle limit and a relatively high zooming ratio.
Japanese Laid-Open Patent Publication No. 2007-140359 discloses a zoom lens, in order from the object side to the image side, having at least two lens units of a negative lens unit, and a positive lens unit. In the zoom lens, intervals between the individual lens units vary at the time of magnification change. Also in the zoom lens, the relationship among a refractive index, a focal length and a radius of curvature of a lens constituting a first lens unit is specified. The zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2007-140359 has a reduced optical overall length and a high optical performance all over the range of magnification change.
Japanese Laid-Open Patent Publication No. 2006-227197 discloses a zoom lens, in order from the object side to the image side, having three lens units of a negative lens unit, a positive lens unit, and a positive lens unit. In the zoom lens, the individual lens units move for magnification change so that an interval between a first lens unit and a second lens unit decrease and an interval between the second lens unit and a third lens unit increase, at a telephoto limit relative to at a wide-angle limit. Also in the zoom lens, the first lens unit is composed of two lenses of a negative lens and a positive lens, the second lens unit is composed of three lenses of a positive lens and a cemented lens composed of a positive lens and a negative lens, the third lens unit is composed of one positive lens, and imaging magnification of the second lens unit at a telephoto limit is specified. The zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2006-227197 has a relatively high zooming ratio with remaining the desired optical performance.
Japanese Patent No. 3589555 discloses a zoom lens, in order from the object side to the image side, having three lens units of a negative lens unit, a positive lens unit, and a positive lens unit. In the zoom lens, a first lens unit moves with locus of a convex to the image side, a second lens unit monotonically moves to the object side from a wide-angle limit to a telephoto limit, and a third lens unit moves with locus of a convex to the object side, at the time of magnification change. Also in the zoom lens, the relationship between a focal length of the entire system at a wide-angle limit and a focal length of each lens unit is specified. The zoom lens disclosed in Japanese Patent No. 3589555 has a reduced optical overall length and suppressed distortion with a small number of lenses.
Japanese Patent No. 3868092 discloses a zoom lens, in order from the object side to the image side, having three lens units of a negative lens unit, a positive lens unit, and a positive lens unit. In the zoom lens, a first lens unit moves with locus of a convex to the image side, a second lens unit monotonically moves to the object side from a wide-angle limit to a telephoto limit, and a third lens unit moves with locus of a convex to the object side, at the time of magnification change. Also in the zoom lens, the relationship between a focal length of the entire system at a wide-angle limit and a focal length of each lens unit is specified. The zoom lens disclosed in Japanese Patent No. 3868092 has a reduced optical overall length and suppressed distortion with a small number of lenses.